Electrostatic spray device and method



june 20, 1967 KlYOSHl INOUE 3,326,182

I ELECTROSTATIC SPRAY DEVICE AND METHOD Filed Dec. 9, 1965 5 Sheets-Sheet 1 KIYOSHI INOUE INVENTOR.

FIG. 2 3

AGENT June 20, E95 KIYOSHI INOUE 3,

ELECTROSTATIC SPRAY DEVICE AND METHOD Filed Dec. 9, 1963 .3 Sheets-Sheet 2- ELECTRONIC SWITCH KIYOSHI INOUE INVENTOR.

(54 AGENT FIG. 3 BY June 20, E967 KIYOSHI INOUE 3,326,182

ELECTROSTATIC SPRAY DEVICE AND METHOD Filed Dec. 9, 1963 5 Sheets-Sheet 15 K/YOSHI INOUE INVENTOR.

- FIG. 5 BY United States Patent 3,326,182 ELECTROSTATIC SPRAY DEVICE AND METHOD Kiyoshi Inoue, 182 S-chorne, Tamagawayoga-machi, Setagaya-ku, Tokyo, Japan Filed Dec. 9, 1963, Ser- No. 329,199 Claims priority, application Japan, June 13, 1963,

8 Claims. (Cl. 118-629) 7 ABSTRACT OF THE DISCLOSURE Apparatus for the electrostatic spraying of a gas stream, comprising a constriction in the path of the gas stream between a high-pressure side and a low-pressure side, means including an electrode at the high-pressure side for applying an electrostatic field along the stream across the constriction whereby an electric discharge is generated in the gas stream at the constriction, and means for introducing particles of a sprayable material into the gas stream in the low-pressure chamber.

My present invention relatesto an apparatus for electrostatic spraying of fluent materials and the treatment (e.g. coating) of surfaces thereby.

In my copending application Ser. No. 302,541 filed August 16, 1963, now abandoned, I disclose a method of and an apparatus for the treatment of surfaces with particulate substances directed thereagainst and electrostatic means for limiting the generation of dust and controlling the configuration of the spray stream. Other electrostatic systems provide for the spraying of fluid materials (e.g. liquid paints and other coating substances as well as particulate material entrained in a liquid or gas stream) whereby uniformity of coverage is assured with only limited waste. In such systems, however, the particles of spray often receive nonuniform and unequal charges and tend to agglomerate, although to a limited extent. In systems of this type, therefore, the high electrostatic potential applied between the spray device and the workpiece frequently is insufficient to assure strong attraction at the workpiece surface for the particles.

It is the principal object of my present invention to provide an apparatus for electrostatically spraying a workpiece surface whereby the aforementioned disadvantages can be avoided.

Still another object of this invention is to provide an apparatus for the treatment of a workpiece surface with a fluid material entrainable in a gas stream whereby approximately uniform electrical charges can be applied to substantially all of the flowable particles which may be liquid, semisolid or solid, as desired.

Yet a further object of my present invention is to provide improved apparatus for the electrostatic coating of workpiece surfaces and adapted to limit agglomeration of the spray substances.

These objects, and others which will become apparent hereinafter, are attained in accordance with the present invention by an apparatus based upon my discovery that a highly uniform distribution of electrostatic charge upon spray particles can be obtained, when the particles are caused to pass through a gaseous, ion-producing electrical discharge during propagation of the particles in the direction of the workpiece surface. It is, therefore, an important feature of the present invention that a gas stream be forcibly directed toward a workpiece surface and a coronatype electrical discharge be established at a point along the path through which the fluid particles, which are injected into the gas stream rearwardly of or at the discharge, must pass. Moreover, I have found that best results are obtained 3,326,182 Patented June 20, 1967 when the gas stream in which the corona-discharge is sustained and which effectively entrains the spray particles in the direction of the workpiece surface has imparted thereto a generally spiroidal (i.e. vortex) motion in the direction of the workpiece about the axis defined by the nozzle of the spray device. Thus, in addition to an axial component of velocity, the gas stream imparted thereto a tangential component serving to create the axially propagated vortex. Surprisingly, it has also been observed that a more uniform charge distribution and a more complete electrostatic charging of the particles can be obtained when the fluid material is discharged axially as particles into the gas stream while a portion of the gas, at least in the region of the corona discharge and the injection of the particles into the gas stream, has a component of movement in the direction opposite to the direction of drift of the particles toward the workpiece.

7 According to a more specific feature of the instant invention, the corona discharge through which the spray particles pass is produced by applying a high-voltage electrostatic field across the path of the gas stream, the latter being pressurized along a rear-most portion of this path and having a reduced pressure along a forward portion of the path; the electrostatic field is such that ionization of the gas is not possible at the pressure maintained along the rearward portion of the path. The pressure is, however, reduced at an intermediate location along the path, the gas expanding at thisv location to produce the corona discharge while atomizing means or the like, for supplying particulate substances to the gas stream, is disposed at this location.

Still another feature of the present invention resides in the provision of means for pulsing the discharge so that a succession of clouds of ionized gaseous particles is formed in the stream, the particulate substance being concentrated in these clouds for charging thereby. Advantageously, the spray devices according to the present invention may include a corona-discharge chamber separated by constriction means from a high-pressure chamber and having a mouth open in the direction of the workpiece to be coated. The supply or atomizing means is then disposed so as to inject the particulate substances into the discharge chamber in which a general drift of the gas and the spray particles is established in the axial direction. The constriction means can be vibrated periodically so as alternately to compress and expand the gas in the region of the constriction means, the gas breaking down i-onically during each expansion movement of the constriction means to produce a cloud or pulse of ionized particles. The discharge chamber can also be provided with a plurality of generally tangential nozzles or passages around the periphery of the chamber for imparting the spiroidal movement to the spray stream; the nozzle means should, however, also be directed somewhat inwardly to ensure complete charging of the particles and reduction in dead spots.

The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

. FIG. 1 is an axial cross-sectional view through a spray device embodying the principles of the present invention;

FIG. 2 is a cross-sectional view taken along the line -IIII of FIG. 1;

FIG. 3 is a view similar to FIG. 1 of a modified spray device with the control and circuit means therefore illus- FIG. 6 is a further axial cross-sectional view through a spray apparatus according to the present invention.

In FIGS. 1 and 2 I show a spray device 10 adapted to apply a coating to the surface 11 of a conductive workpiece which is grounded via lead 12. The spray device 10 comprises a generally cylindrical housing or shell 13 forming with a re-entrant cylindrical portion 14 a pressure compartment 30 to which a gaseous fluid is fed via inlet 29. The re-entrant portion 14 forms a compartment 17 through which an axial flow of the gas is induced past an atomizing nozzle 19 to which the liquid to be supplied is fed by a supply tube 20. The latter passes through pressure-retaining seals 21, 22 in the housing portions 13 and 14 and is provided with a throttle valve 23 adapted to adjust the liquid flow rate. A pump 24 forces the liquid through the supply conduit to the atomizing nozzle 19. An outwardly converging annular opening 18 surrounds the nozzle 19 and communicates between compartment 17 and the corona-discharge chamber 26 to provide the gas necessary to atomize the liquid and carry it axially in the direction of the workpiece surface 11. An inlet 16 in re-entrant portion 14 communicates between the pressure chamber 13 and compartment 17. The re-entrant portion 14 defines with the housing 13 an annular axially extending passage 15 which supplies fluid under pressure to the nozzle-like apertures 25. The latter constitutes constriction means together with passage 18 at which expansion of the gas from chamber 30 takes place to generate the electrical discharge.

Apertures 25 are inclined to the radial direction and thus impart a generally tangential component to the gas introduced through these apertures into low-pressure discharge chamber 26. The spray from nozzle 19 thus passes through the discharge generated at the chamber 26 as it is propagated in the axial direction (arrow 31) and is displaced spiroidally in a vortex as a consequence of the inclination of apertures 25. The high electrostatic field (between, say, 500 and 2000 volts/cm), applied across the path of the gas stream, is developed between the workpiece surface 11 and an electrode plate 27 mounted within chamber 30 of housing 13. A high-voltage source 32 has its negative terminal connected to electrode 27 via a conductor 33 passing through an insulator 28. In a system of this type a substantially uniform charge distribution is applied to the spray particles as the latter pass through the chamber. In general, it may be noted that apertures 25 will introduce ionized particles into the discharge chamber 26 and the electrostatic field will result in a polarization of ions within this chamber with an accumulation of negative ions in the region of the mouth thereof and an accumulation of positive ions in the corner regions of this chamber. This concentration of negative ions at the mouth of the chamber results in the application of highly negative charges to the particles which are then attracted strongly to the oppositely charged workpiece surface.

In the device 50 of FIG. 3, the housing 90 contains a blower 51 which is driven by a motor 53 and has flexible vanes 52 for displacing air or some other ionizable carrier gas axially through the discharge chamber 57. The latter is formed with an apertured screen 58 constituting an oscillating diaphragm adapted alternately to increase and decrease slightly the pressure within the blower chamber 91 so that, upon each movement of the diaphragm screen 58 to the left, the gas in the region of the screen and expanding through the apertures thereof will ionize to produce successive clouds 76 of ionized gas particles. The diaphragm screen 58 is mounted upon a bearing 60 in the housing 90 for oscillating movement by means of a pair of electromagnets 63, 64 which alternately attract the magnetizable armature 62 at the end of an actuating rod 61 reached with the screen. A capacitor 65 is connected in circuit with one of the electromagnets so that, upon energization of both electromagnets by the alternating current source 71, one of the magnets will attract the armature 62 in phase within alternating current pulse while the other electromagnet operates out of phase therewith to shift the diaphragm in the opposite direction.

A nozzle 59 introduces the liquid to be spread into each cloud of ionized particles and thus swings in the cadence of diaphragm screen 58. For most effective results, the nozzle should be spaced from the screen by a distance equal to an integral number of half-wave lengths of the frequency of oscillation of the screen and nozzle. The oscillation frequency should range between 10 and 5000 cycles/sec. with a gas velocity between 10 and 50 m./ sec. Nozzle 59 is mounted in a bearing 66 and provided with an armature 67 alternately attractable by electromagnets 68, 69, one of which is connected in circuit with a phaseshifting capacitor 70 for the purpose previously described. Since the nozzle 59 swings in the cadence of diaphragm 58, the nozzle will remain in a cloud of ionized particles for relatively prolonged periods to concentrate the liquid in such clouds. While nozzle 59 is shown to be provided with a supply tube 74 and a flow-control valve 73, of conventional type, the valve may be electromagnetically operated in the cadence of alternating-current source 71 to pass liquid into the discharge chamber 57 only when the nozzle is surrounded by a cloud of ionized particles.

In the embodiment of FIG. 3, the electrode in the highpressure chamber 91 is constituted by the conductive blower rotor 51 whose shaft 54 is engaged by a brush or wiper 55 connected to the negative terminal of a battery 75. The electrostatic field is augmented by the field of a high-voltage transformer 82, energized by an AC. source 83 and connected via rectifier 81 between the positive terminal of battery 75 and ground. The positive terminal of battery 75 is tied to the mouth-forming head 79 of discharge chamber 57. This head is connected by conducters 80 to a discharge-augmenting element 77 provided with a multiplicity of pointed members 78 at which a corona discharge is sustained. As the ion clouds 7 6 and the flowable material entrained thereby pass through the corona discharge at member 78, any undischarged particles of the sprayed material are electrostatically charged for displacement toward the workpiece surface 84 which is grounded. Source 71 activates generator 83 via an electronic switch 72 so that the corona discharge at member 78 is pulsed in the cadence of oscillation of diaphragm 58. When the body 77 is dispensed with, a corona discharge develops at the outer edge 93 of head 79.

In the system of FIG. 4, which is used to coat the mold cavity of a die 101 grounded by lead 102, the liquid material (e.g. protective resin) is supplied to the central bore 103 of a spray device in the direction of arrow 106, a valve 104 controlling the rate of flow through the tubular shank 105 of this housing. From inlet bore 103, the liquid passes through the ratomizing nozzle 107 into the discharge chamber 119 in which a corona discharge is sustained as previously described. The carrier gas is introduced via conduit 108 and a throttle valve 109 into a tube 110 which supplies the air to a manifold chamber 111; from this chamber a plurality of bores 112 convey the carrier gas to a collecting chamber 113. The annular chamber 113 communicates with an annular passage 114 surrounding the re-entrant portion 116 of the housing and opening into the pressure compartment 118 in which is disposed an electrode 120. The latter is connected via a lead 123 in an insulating sheath 121 to the negative terminal of a high-voltage source 122 whose positive terminal is grounded via a conductor 124. A plurality of rearwardly and tangentially oriented passages 117 connect the high-pressure chamber 118 with the discharge chamber 119 and constitute constriction means at which the corona discharge is developed. Another portion of the carrier gas passes from compartment 113 through bores 125 to a similar compartment 126 surrounding the nozzle 107 and communicating with chamber 119 via a similar opening 127 to provide the drift of gaseous medium in the direction of workpiece 101. The tangential orienta-.

tion of the passages 117 insures a generally spiroidal movement of the spray-entraining gas stream while the rearwardly directed flows from the passages insure thorough electrostatic charging of the spray,

The spray device 140 of FIG. 6 comprises a reservoir 141 for the liquid 142 to be sprayed; a valve 143 is reciprocable to alternatively cut off and permit the flow of liquid to the discharge body 150 formed with the atomizing nozzle 148. A needle valve 151 is provided to regulate the rate of flow of this spray liquid which is entrained as a consequence of the Venturi effect by air under pressure passing through the re-entrant portion 152 of the housing via passages 153 and 149. Again the reentrant portion 152 forms an annular passage 147 from which gas expands at the inwardly and tangentially directed sets of apertures 145 and 146 into the discharge chamber 163. The pressure chamber 154 is supplied by a blower 161 in a housing 160 via baflles 155 and 156 preventing undesirable laminar-flow currents from passing through the device without interruption. The electrode means in the pressure chamber 154 is constituted by screens 158 mounted in an insulated portion 157 of the housing. The negative terminal of the high-voltage DC source 159 is connected to this electrode means while the positive terminal is grounded together with the workpiece 162. This device functions in a manner similar to those previously described. In general, it may be noted that, for the purposes of the present invention, it is desirable to maintain a gas pressure of substantially 2- 5 kg./cm. in the pressure chamber and to insure that the pressure drops to substantially 0.8-1 atm. in the discharge chamber.

Example I Using a device of the type illustrated in FIG. 4, having an air nozzle 127 with an outer diameter of 2 mm. and a liquid nozzle of 0.8 mm. inner diameter, and 6 mm. length, a melamine-resin paint of sprayable consistency was directed against a surface 65 cm. away from the mouth of the discharge compartment 119. The carrier gas was compressed air and a pressure of 3 kg./cm. was generated in the pressure compartment 118 while the paint was applied at a rate of 100 cc./min. with an average particle size of 1 micron and a spray cone having an apex angle of 80. An air velocity of 20 m./sec. was employed. When no electrostatic field was supplied, only 65% of the paint was deposited upon the plate. When, however, a field of 80,000 volts was applied between electrode 120 and the workpiece 101, it was found that 90% of the paint adhered to the surface and the latter had a uniform coverage. With this electrostatic field a corona discharge was observed at the mouth of and within the discharge chamber 119.

Example 11 Using the apparatus and conditions of Example I except that the compressed air within the pressure chamber was at 4 kg./cm. pressure, the paint particle size was 0.8 micron and the spray cone had an apex angle of 70 with a spray head-to-workpiece distance of 80 cm., the absence of an electrostatic field gave 80% transfer of paint to the workpiece while the use of a field of 80 kg. voltage produced 98% transfer.

Under the conditions of Example I except that the paint particle size was 6 microns, the spray cone had an apex angle of 65 and the workpiece was 90% away from the spray head, the use of air at 5 kg./cm. resulted in a 98% transfer of paint to the workpiece surface in a smooth layer when a field of 80 kilovolts was applied. In the absence of this field, a coverage of 90% was obtained. In general, when the system of FIG. I was employed and the air nozzles 25 were not rearwardly directed, a coverage of 95% was obtained with the electrostatic field as compared with 98% when the apertures 117 were both tangentially and rearwardly directed. In the latter case, eight apertures 117 were provided with bores of 0.6 mm., the apertures treminating in the discharge chamber at a distance of 1 mm. from the rear wall of this chamber which had a diameter of 6 mm. The apertures 117 were inclined rearwardly at an angle of 18 to a plane perpendicular to the axis of the nozzle and were inclined at 25 to the corresponding radius.

The invention as described and illustrated is believed to admit of many modifications within the ability of persons skilled in the art, all such modifications being considered within the spirit and scope of the appended claims,

What is claimed is:

1. An electrostatic spray device, comprising housing means for directing a gas stream toward a surface to be sprayed and including a high-pressure chamber and a low-pressure chamber open in the direction of said surface and disposed forwardly of said high-pressure chamber in the direction of flow of said gas stream; constriction means communicating between said chambers for expanding gas of said stream upon passage thereof from said high-pressure chamber to said low-pressure chamber; means including an electrode in said high-pressure chamber for applying an electrostatic field along said stream and across said constriction means whereby an electrical discharge is generated in said gas stream at said constriction means; and supply means for dispensing particles of a sprayable material into said gas stream in said low-pressure chamber.

2. A device as defined in claim 1, further comprising means for periodically oscillating said constriction means generally in the direction of fiow of said gas stream for producing a succession of ion clouds in said stream within said low-pressure chamber.

3. A device as defined in claim 2 wherein said supply means includes at least one nozzle downstream of said constriction means and oscillatable in cadence there with for feeding said particles into said clouds.

4. A device as defined in claim 1, wherein said constriction means includes an array of angularly spaced generally tangential gas passages surrounding said lowpressure chamber and communicating between said lowpressure chamber and said high-pressure chamber for imparting a generally spiroidal movement to said gas stream in the direction of said surface.

5. A device as defined in claim 4, wherein at least some of said air passages are inclined rearwardly of the direction of displacement of said gas stream.

6. A device as defined in claim 4 wherein said supply means includes a central nozzle for injecting a stream of liquid into said low-pressure chamber in the direction of said surface, said constricting means further comprising an annular gas passage surrounding said nozzle.

7. A device as defined in claim 6 wherein said lowpressure chamber is defined 'by an axially extending reentrant portion of said housing means open in the direction of said surface.

8. A device as defined in claim 1 wherein said means for applying an electrostatic field along said stream includes a first electrode disposed in said high-pressure chamber and a source of high-voltage direct current connected between said first electrode and a second electrode constituted by said surface.

References Cited UNITED STATES PATENTS 3,082,956 3/1963 Point 118629 X 3,049,301 8/1962 Heuschkel 23915 2,894,691 7/1959 Sedlacsik 118-621 X CHARLES A. WILLMUTH, Primary Examiner. PETER FELDMAN, Examiner. 

1. AN ELECTROSTATIC SPRAY DEVICE, COMPRISING HOUSING MEANS FOR DIRECTING A GAS STREAM TOWARD A SURFACE TO BE SPRAYED AND INCLUDING A HIGH-PRESSURE CHAMBER AND A LOW-PRESSURE CHAMBER OPEN IN THE DIRECTION OF SAID SURFACE AND DISPOSED FORWARDLY OF SAID HIGH-PRESSURE CHAMBER IN THE DIRECTION OF FLOW OF SAID GAS STREAM; CONSTRICTION MEANS COMMUNICATING BETWEEN SAID CHAMBERS FOR EXPANDING GAS OF SAID STREAM UPON PASSAGE THEREOF FROM SAID HIGH-PRESSURE CHAMBER TO SAID LOW-PRESSURE CHAMBER; MEANS INCLUDING AN ELECTRODE IN SAID HIGH-PRESSURE CHAMBER FOR APPLYING AN ELECTROSTATIC FIELD ALONG SAID STREAM AND ACROSS SAID CONSTRICTION MEANS WHEREBY AN ELECTRICAL DISCHARGE IS GENERATED IN SAID GAS STREAM AT SAID CONSTRICTION MEANS; AND SUPPLY MEANS FOR DISPENSING PARTICLES OF A SPRAYABLE MATERIAL INTO SAID GAS STREAM IN SAID LOW-PRESSURE CHAMBER. 